Animal domestication can be thought of as developing a mutually useful relationship between animals and humans. Over the past 12,000 years, humans have learned to control their access to food and other necessities of life by changing the behaviors and natures of wild animals. All of today's domesticated animals—including dogs, cats, cattle, oxen, llamas, sheep, goats, camels, geese, horses, chickens, turkeys, and pigs—started out as wild animals but were changed over the centuries and millennia into animals that are tamer, quieter, and generally more cognitively suited to a lifestyle of coexistence with humans. Today people benefit from domesticated animal in many ways including keeping cattle in pens for access to milk and meat and for pulling plows, training dogs to be guardians and companions, teaching horses to adapt to the plow or take a rider, and changing the lean, nasty wild boar into the fat, friendly pig.
When individuals are looking to breed animals, they look for certain traits in purebred stock that are valued for a particular purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different, and, it is presumed, superior abilities in a given area of endeavor. For example, to breed chickens, a typical breeder intends to receive eggs, meat, and new, young birds for further reproduction. Thus, the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. On the other hand, purebred breeding aims to establish and maintain stable traits that animals will pass to the next generation. By “breeding the best to the best,” employing a certain degree of inbreeding, considerable culling, and selection for “superior” qualities, one could develop a bloodline superior in certain respects to the original base stock.
As first noted by Charles Darwin, domesticated animals are known to share a common set of physical characteristics, sometimes referred to as the domestication phenotype. C. Darwin, THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION (2nd ed.) (New York: D. Appleton & Co., 1883). They are often smaller, with floppier ears and curlier tails than their untamed ancestors. Their coats are sometimes spotted while their wild ancestors' coats are solid. One long-term study demonstrating this phenomenon has been ongoing since 1958 at the Institute of Cytology and Genetics in Novosibirsk, Russia. In this study, scientists have successfully demonstrated that, through careful selective breeding for tamability, wild Siberian silver foxes acquire both the behavioral and appearance traits of domesticated dogs. See, e.g., L. Trutt, Early Canid Domestication: The Fox Farm Experiment, 87 AMERICAN SCIENTIST 160-69 (March-April 1999). This highly conserved combination of psychological and morphological changes during the process of domestication is seen to varying degrees across a remarkably wide range of species, from horses, dogs, pigs, and cows to some non-mammals like chickens and even a few fish. However, in no other species has this relationship between behavior and anatomical features been more widely noted than in the horse.
Relationships also exist in humans between physiological feature sets (i.e., phenotypes) and certain cognitive functions and/or personality traits. During progressive stages of human embryonic growth, development of the brain and face remains intimately connected through both genetic signaling and biomechanical/biochemical mechanisms. The face develops from populations of cells originating from the early neural crest, with cells from the neural tube gradually shifting to form the prominences of the face. Differentiation of these early cells is closely regulated through intricate genetic signaling mechanisms, with the brain essentially serving as the platform on which the face grows. As these two structures continue to grow and develop during the later embryonic stages, their phenotypes remain closely linked as complex genetic hierarchies regulate patterns of cross talk between molecules, cells, and tissues.